This invention relates to a computer system having a host computer and a storage system. More specifically, this invention relates to a technique of switching logical paths that connect a host computer and a storage system.
There have been known multi-path computer systems in SAN (Storage Area Network) environments. A multi-path computer system has a storage system and a host computer, which are connected to each other by a SAN containing a Fibre Channel switch.
A storage system in a multi-path computer system provides a logical unit, which is connected to a host computer via a plurality of logical paths. The logical paths are paths provided for redundancy in accordance with combinations of physical paths along communication routes between a host computer and a storage system. The physical paths are I/O paths connecting the host computer and the storage system to each other. The I/O paths are, for example, SCSI cables or Fibre cables.
Generally speaking, a plurality of logical paths pass common components. Accordingly, when a failure is detected in one logical path, there is a strong possibility that other logical paths are also experiencing a failure. In the case of a failure in a Fibre Channel switch, for example, every logical path that passes this Fibre Channel switch suffers a failure.
A host computer in a multi-path computer system needs to choose which logical path is to be used for transmission of an I/O request to a logical unit set in a storage system.
A technique of selecting a logical path by round robin is disclosed in JP 2004-185093 A. According to this technique, the host computer chooses a logical path by round robin and uses the chosen logical path in sending the I/O request.